An adequate supply of drinking water that is free of carcinogens, bacteriophages, and other harmful microorganisms, has been and continues to be a major public health issue.
Although there are several methods known for obtaining potable water, the use of chlorine has been the predominate disinfection agent of choice during the past century. Chlorine is generally used to disinfect tap water, but its propensity for generating carcinogenic by-products such as trihalomethanes has become a serious health problem.
Other treatment methods including the application of ozone or thermal energy are either potentially harmful to the environment or cost ineffective. The application of ultraviolet light, on the other hand, has been demonstrated to be an effective disinfection agent, and in certain cases more effective than chlorine, particularly in the cases of viruses. Ultraviolet radiation has the singular attribute of creating no known toxins.
Whereas the use of chemicals and ozone as a bactericide involves penetration through its cell wall to reach various cellular components, exposure to ultraviolet light does not cause cell death but rather it produces a photo-induced transformation of a cell's nucleic acids causing sterility. Without a reproductive ability, the bacteriophage is not considered to be infectious. The optimum illumination time and light intensity, measured in millijoules/cm.sup.2, that is required to sterilize the bacteriophage has been experimentally determined. Any viable ultraviolet disinfection system must exhibit a predetermined exposure (the product of irradiance and time) threshold based on the turbidity factor of water.
The prior art shows many examples of the treatment of water with ultraviolet light. Perhaps the best example in the prior art may be found in about five percent of municipal water treatment plants in the United States. These water treatment plants typically employ an in-ground facility featuring an open channel or sluice (See FIG. 1). Said structure, by virtue of its confines, constricts the flow of water to favorable values of depth and width and rate of flow.
Into this volume of water a plurality of low-pressure mercury discharge lamps 10, each encapsulated within transparent quartz tubing, are immersed in a vertical and perpendicular-to-flow configuration. Said plurality of lamps is arranged into vertical modules placed transversely in the sluice 12 in a uniform array or bank covering both the width and depth of the channel. Typically, there are several lamp banks positioned uniformly in rows along the channel. The required ultraviolet dosage threshold for the water effluent quality determines the number of vertical lamps per channel. This arrangement resembles a sieve through which the water must flow. The angular light distribution of the light emanating from each lamp fills the void between lamps and between rows; irradiating the water as it passes through said voids or interspaces. As the water flows past lamp modules, a sufficient dose is achieved to render the bacteriophage impotent.
Occasionally a horizontal and parallel-to-flow open channel lamp configuration is employed (See FIG. 2). In this configuration, a plurality of lamps 20 is arranged into horizontal modules in a uniform vertical array. The water depth establishes the number of lamps per module. As in the case of the vertical and perpendicular-to-flow configuration, each lamp is encapsulated within transparent quartz tubing. Water flows along the interspaces between lamps 22, experiencing radiation along the length of the low-pressure mercury discharge lamps.
There is yet another example of prior art, shown in FIG. 3, in which a plurality of lamps 30, each encapsulated within transparent quartz tubing, is positioned longitudinally in a horizontal and parallel-to-flow configuration within a cylindrical shaped closed vessel 32. The number of lamps is determined by the inner diameter of the closed vessel. Not unlike the previous example, water flows into the vessel through an inlet pipe 31 receiving radiation as passing along the length of the pipe, exiting as safe water at exit pipe 33. In this prior art example, the device is intended for application where the volume of water to be treated is less than that normally required of a municipal water plant. Said device might satisfy, for example, the needs of a large commercial bottling enterprise.
These prior art examples, despite their demonstrated utility, have several significant drawbacks. First and foremost, the water-contacting surface of the quartz tubing, which isolates the lamp from the water, is prone to an accumulation of particulate matter, salts, minerals and other contaminants (scaling buildup) found in the water. The lamp exitance may be sufficiently absorbed by the inorganic scaling to reduce irradiance below the dose threshold required for sterilization. Second, the scaling buildup on the quartz tubing must be frequently removed requiring heavy maintenance.
In said horizontal and vertical lamp configurations, the cleaning of the lamp modules generally means that the lamp modules must be removed from the channel of chamber (See FIG. 1). A bridge crane 14 is used to carry the lamp modules from the disinfection channel into an acid tank 16 for cleaning. The acid tank must, in turn, be cleaned and the residue discarded in, for example, the disinfection plant's headwaters. Third, ultraviolet light does not transmit efficiently in water even through nominal distances, requiring the separations between lamps to be kept small. As a concomitant result, a large number of lamps are required to affect sufficient dosage for bacteriophage sterilization. Finally, there is a danger of electrical shock owing to the aggregate voltage supplied to the plurality of lamps that must be delivered through immersed electrical conduit.
Thus, a need has arisen for an ultraviolet water disinfection system that avoids the aforementioned drawbacks, particularly, with regard to providing for a longer term, stable level of illuminance consistently sufficient to sterilize bacteriophages.